The present invention relates to a fuel injection system of an internal combustion engine, in which the amount of fuel injected by a fuel injection valve, hereforth referred to as the fuel injecting amount, is determined based on a physical model describing a behavior of fuel coming into a cylinder of the engine.
A fuel injection system is disclosed, which determines a fuel injecting amount of a fuel injection valve so that an air/fuel ratio of an air/fuel mixture supplied to an engine is adjusted to coincide with a target ratio, for example, in Published Unexamined Japanese Patent Application No. 59-196930. The system uses identification that the linear approximation holds between a control input and a control output. The control input is regarded as a compensation value for compensating a basic fuel injecting amount obtained from the rotating speed of an engine and the amount of intake air. The control output is regarded as an actual measurement of the air/fuel ratio detected by an air/fuel ratio sensor. Using such identification provides a physical model for describing dynamic behavior of the engine, based on which a control law is designed. The system of this known type, based on the linear control theory, is thus constructed to determine the fuel injecting amount, utilizing the control law.
Actually, however, the linear relationship does not hold between the control input and the control output. The physical model obtained from a simple linear approximation, thus, is allowed to describe the dynamic behavior of the engine accurately only in a very limited operating condition. For this reason, the conventional systems suppose several physical models in several regions of the engine operation in each of which the linear approximation can almost hold. Accordingly several control laws corresponding to the physical models must be designed in respective regions. In the aforementioned system, control laws have to be switched depending on the physical model in the respective region of the engine operation, resulting in cumbersome control. Switching the control law might cause the control at the boundary between the regions to be unstable.
A system of this type uses an approximation by lower order physical model for improving responsiveness of the control by reducing calculating time. In this method, an approximation error or an error due to the difference among individual engines is absorbed by an integral operation. However, in the conventional method, the physical model is provided based on physically meaningless state variables on the assumption that the linear approximation can hold between the control output and control input. Hence approximating the physical model by lower order will deteriorate the control accuracy because of the increase in the amount of the integral term.
Further, since the above system determines the fuel injecting amount in accordance with an actual measurement of an air/fuel ratio detected by an air/fuel ratio sensor as the control output, the control cannot be applied to an engine with no such sensor.